Sera which contain anti-insulin receptor antibodies were used as probes of insulin receptors in rat adipocytes, human fibroblasts, and rat muscle. These sera block insulin binding, and acute exposure had insulin-like biological effects. This activity resided in the F(ab'2) portion of the molecule. Fab' fragments bind to receptor, block insulin binding and action, but are without bioactivity. These findings suggest an important role for receptor aggregation in insulin action. The Fab' also provides the first competitive inhibitor of insulin action at the receptor level.